An ulvophycean marine green alga produces large parthenogenetic isogametes as predicted by the gamete dynamics model for the evolution of anisogamy

In eukaryotes, the gamete size difference between the two sexes
(anisogamy) evolved from gametes of equal size in both mating types
(isogamy). The gamete dynamics (GD) model for anisogamy evolution combines
gamete limitation and competition and predicts that, if gametes of both
mating types can develop parthenogenetically (i.e. without fusing with the
opposite mating type), large isogamy can evolve under gamete-limited
conditions. Ulvophycean marine green algae that have been claimed to
exhibit various gametic systems from isogamy to anisogamy are important
models for testing such theories. However, in most previous papers,
whether a species is isogamous or anisogamous has not been examined
statistically. Caution is necessary regarding claims of slight anisogamy
because of gamete size variation. We reveal (i) that the gametic system of
Struvea okamurae is large isogamy using a generalized linear mixed model
(GLMM), which accounted for the variation of gamete size among individual
gametophytes, and (ii) that gametes of this alga can actually develop
parthenogenetically, contrary to a previous report. Its habitat
environments and protracted duration of gamete release suggest that this
alga might experience gamete-limited conditions. S. okamurae
seems to produce large parthenogenetic isogametes following GD model
predictions, as an adaptation to deep waters.